A known method of calibrating an array antenna is to use calibration coupler manifolds 150, as shown in FIG. 1, at each of the elements 140 in the array.
Referring to FIG. 1, there is shown a known antenna element comprising a receiver 110, array cabling 120 and various active components 130. A calibration signal from a central source is split many ways in the manifold and a nominally-equal proportion is coupled into each element channel at some point behind the radiating element. The signal level at the receiver(s) 110 can then be adjusted accordingly to produce the desired performance characteristics for the array antenna.
When using a calibration coupler, a portion of the element channel 140 is not included in the calibration process. One problem with calibration coupler manifolds 150 is that they are relatively large devices and so cause problems in the design of an array antenna which incorporates them. Another problem with calibration coupler manifolds 150 is that the coupling factors at each channel have individual variability which needs to be removed to achieve optimum performance, i.e. the accuracy of antenna calibration is limited to the extent that the individual manifold outputs are known.
Alternatively, another known method for calibrating an array antenna is to use an external scanner. This involves placing an external scanning apparatus in front of the array face and scanning the properties of each radiating element of the array in turn by moving the scanner over each radiating element and measuring the radiation it produces and/or receives. It has many moving parts which require maintenance, especially because the equipment usually operates in exposed environments as this is where equipment employing phased array antennas is usually operated. In addition, this is a slow process and requires normal use of the equipment to stop while calibration is performed.